This application is related to Japanese applications Nos. Hei 08(1995)-288205 and Hei 9(1997)-214824 filed on Oct. 30, 1996 and Aug. 8, 1997, respectively, whose priorities are claimed under 35 USC .sctn. 119, the disclosures of which are incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a permeable adhesive tape and a process for preparing the same, more particularly a permeable adhesive tape taken up in a roll whose permeability never or hardly deteriorates with time and a process for preparing the same.
2.Description of Related Art
Conventionally used adhesive tapes such as sticking plasters, bandages and surgical cloths are adhered on skin in use. When adhered and maintained on skin for a long time, the adhesive tapes may cause a rash, an eruption or maceration on skin. For avoiding this problem, it is desirable that the adhesive tapes have permeability.
U.S. Pat. No. 4,789,413 discloses a process for preparing an adhesive tape having permeability, which process includes dispersing a water-absorptive high-molecular weight substance containing water uniformly in a rubber-base or acrylic pressure sensitive adhesive solution in an organic solvent, applying this dispersion onto a release sheet, followed by drying, and then laminating a porous backing onto the dispersion-applied surface.
In general, the adhesive tape is wound up on a bar core into the form of a roll when manufactured in a factory, and then stored or shipped in the form of the roll before processed into a desired shape.
Conventionally the tape wound up in a roll receives a considerable winding pressure. (Here, the winding pressure means a force acting on an adhesive tape wound on a bar core in a roll.) Especially, the winding pressure is considerably high around the center of the roll. This winding pressure may fluidize the pressure sensitive adhesive in the adhesive tape, which may then fill permeable pores of the adhesive tape, most remarkably around the center of the roll. Thus the adhesive tape gradually loses its permeability with time. Furthermore, after aging, the adhesive tape is cut to a width of a desired product, and also wound up in a small roll, which is called a product roll, for convenience of storage. Since the tape in the form of the product roll also receives the winding pressure, it is impossible to prevent deterioration of permeability with time even of the product roll.
For preventing or easing the deterioration of permeability, it is known to wind a paper tube (a core) with polyethylene foam or the like and take up the adhesive tape on the paper tube so that the winding pressure is relaxed. Also it is known to take up the adhesive tape less tight at a controlled wind-up tension.
However, in the former way, the polyethylene foam must be formed thick to sufficiently relax the winding pressure, and the thick foam may cause deformation of the tape. In the latter way, the loosely taken-up roll may lose its shape easily.
It is also known to add hydrophobic inorganic particles such as silica into the pressure sensitive adhesive layer from U.S. Pat. No. 5,198,064. The hydrophobic inorganic particles described in this publication, however, are added not for the purpose of ensuring the permeability but for the purpose of preventing contact of water contained in a permeability-providing agent with a crosslinker added for improvement of the adhesion of the pressure sensitive adhesive layer. Therefore, it is impossible to prevent the permeability from deteriorating due to the winding pressure even by adding the hydrophobic inorganic particles.
Further it is known from Japanese Unexamined Patent Publication (Kokai) No. Hei 2(1990)-45582 to apply an adhesive liquid containing hollow microspheres onto a permeable backing with pores having an average pore diameter of 1 to 50 .mu.m, and partially remove the self-adhesive liquid by gas blowing or air suction through the pores of the permeable backing so as to form a permeable adhesive layer containing pores. However, the size of pores formed by gas blowing or air suction is generally small in correspondence with the pores of the permeable backing, and therefore sufficient permeability cannot be obtained in the adhesive layer. Furthermore, in the case where the pores are formed by gas blowing, the surface of the adhesive layer is roughened. In the case where the pores are formed by air suction, on the other hand, the adhesive liquid leaks out onto a surface of the permeable backing opposite to the adhesive-applied surface.